What to put on kwila decking, and why it bleeds red

Put a penetrating decking oil or a stain made for hardwoods on kwila, after the boards have weathered for at least a few weeks and been scrubbed with a timber and deck wash. It bleeds red because kwila is loaded with water-soluble tannin that rain flushes out of the timber for months, and that runoff stains concrete, tiles and pavers wherever it lands.

Neither of those is a defect in the timber. They are just how kwila behaves, and the sensible approach is to plan around it rather than fight it.

Why does kwila bleed red?

Kwila contains a water-soluble tannin that leaches out quickly whenever the boards get wet, whether from rain or the hose. Resene's decking guide says plainly that this tannin will stain concrete and tiles. Timber supplier Abodo notes that the leaching can continue for months with high-tannin species like kwila, and that most of it comes out through the ends of the boards, because the end grain lets water run through the timber and carry the extractives with it.

That is why the worst staining usually sits below a step edge, a deck perimeter or the cut ends of a bench: that is where water leaves the timber.

Washing helps but does not finish the job. Abodo's view is that deck washes and tannin removers reduce the bleed rather than eliminate it, so expect some colour in the runoff even after good preparation, especially through the first wet season.

How long should new kwila weather before you coat it?

New kwila has a dense, oily surface that stain struggles to get into. The manufacturers' own figures vary depending on which document you read, which is worth knowing before anyone tells you there is one correct answer:

• Resene's decking steps say to let new kwila weather for 4-5 weeks if practical, and its notes on the same page say ideally 4-6 weeks.
• Resene's deck staining project sheet recommends hardwoods be left 8-12 weeks so the grain starts to open up.
• The Cabot's oil-based deck stain data sheet refers to a 4-6 week weathering period for new timber, which it says can be skipped by using its new timber prep product first.

Our practical reading: a month is the minimum, and two to three months is better if you can live with the look. Whatever the wait, the boards then need scrubbing with a timber and deck wash, a thorough rinse and time to dry. Resene notes that washing the tannin out usually takes 2 or 3 washes.

What should you actually put on it?

There are three sensible choices, and one honest non-choice. The trade-off is mostly between how natural it looks and how often you are back on your knees.

Finish	What it does	Maintenance commitment
Clear decking oil	Non film-forming, gives some UV and mould protection, keeps the timber looking natural	Resene says expect an annual application
Hardwood (kwila-tone) stain	Adds red-brown pigment to hold the kwila colour as the natural tannin fades	Resene allows for a second application a few months after the first (3-4 months in its steps), then ongoing recoats
Solventborne pigmented deck stain	Changes the colour, more pigment means more UV protection than a clear oil	Recoat as it wears; traffic lanes go first
Nothing	Kwila silvers off to grey as the surface weathers	A wash now and then; accept the grey

Two product details matter here. Resene's kwila-specific stain is designed for hardwoods and is not suitable for pine, so do not buy one tin for a mixed pine and kwila area. And Resene's notes on kwila point to its hardwood stain and solventborne stain as the products that absorb more readily and last longer on weathered kwila, which is another reason not to rush the weathering stage.

Most people choose kwila for the look of the timber, so we would not paint it. If you want a painted deck, it is cheaper to start with a timber that suits paint. Our piece on whether to paint or stain a deck covers that trade-off.

Why does the colour fade so fast after the first coat?

This catches people out every year. The deep red colour of new kwila is largely the tannin itself, and as rain leaches it out the timber fades quickly. Resene's notes say its kwila stain helps retain the original colour, but the kwila underneath will fade anyway and the stain needs a further application within months, not years. Budget for two coats in the first year rather than assuming the first one will carry you through.

After that first year, things settle down. Treat the deck the same as any stained deck: wash it, check whether water still beads on the surface, and recoat when it stops beading.

How do you stop kwila staining the concrete?

You reduce it rather than stop it. The things that actually help:

1. Weather and wash the boards before new paving or concrete goes in underneath or beside the deck, if the build order allows.
2. Protect adjoining concrete, pavers and tiles with drop sheets while washing, and rinse runoff away before it dries.
3. For kwila outdoor furniture, Resene's guide says manufacturers recommend leaving it on grass to weather before it goes onto a patio or deck.
4. Look at where water leaves the deck. A board end or step nosing that drips straight onto a light-coloured path will keep staining it every time it rains, so redirecting that drip line is worth more than any cleaner.

For stains already on concrete, ask your paint or hardware supplier for a cleaner made for tannin marks on masonry and try it on a hidden corner first. Some old stains lighten a lot, some only partly, and nobody can promise which yours will be.

Is this a job you can do yourself?

Yes. Oiling or staining a kwila deck is one of the more DIY-friendly exterior jobs: wash, scrub, rinse, dry, then apply along the length of the boards with a deck brush or pad, working the edges and ends. Choose a dry spell and check the tin for the temperature range it needs.

Where a professional earns their money is on large or multi-level decks, decks mixed with painted balustrades or cladding that need protecting, and old decks that have been coated with several different products over the years and need assessing before anything goes on. If you want to see colour options first, our deck stain colour guide shows how different stains read on timber, and our wood staining service covers kwila, pine and cedar decks across the region.

If you would rather hand it over, or want someone to look at a deck that is already bleeding onto new paving, get in touch for a quote.